Programmable Unijunction Transistor (PUT) Tutorial: Using the onsemi 2N6027
The Unijunction Transistor (UJT) was once a cornerstone of timing and triggering circuits, famously used in oscillators and SCR control. However, its popularity waned due to limitations like low input impedance and limited parameter adjustability. Its modern, more versatile successor is the Programmable Unijunction Transistor (PUT). This active device offers superior control and flexibility, making it ideal for a wide range of applications. This tutorial explores the fundamentals of the PUT, focusing on the practical use of the onsemi 2N6027, a classic and widely available PUT model.
What is a Programmable Unijunction Transistor (PUT)?
Despite its name, a PUT is not a unijunction transistor at all. It is actually a four-layer (PNPN) thyristor with an anode, a cathode, and a gate. Its operation is functionally similar to a conventional UJT, but its critical characteristic—the intrinsic standoff ratio (η)—is not fixed by its internal construction. Instead, it is "programmed" by two external resistors. This allows designers to easily set the trigger voltage to their exact requirements.
Basic Operation and the 2N6027
The onsemi 2N6027 is a three-terminal PUT in a TO-92 package. Its terminals are:
Anode (A)
Cathode (K)
Gate (G)
The device remains in its OFF state until the anode voltage (`V_A`) exceeds the gate voltage (`V_G`) by approximately 0.7 V (the forward voltage drop of the internal gate-cathode PN junction). When this condition is met, the device triggers and switches to a low-impedance ON state, allowing significant current to flow from anode to cathode. It stays on until the anode current falls below a certain threshold, known as the valley current (`I_V`), at which point it resets to its OFF state.
A Classic Relaxation Oscillator Circuit
The most common application for a PUT is a relaxation oscillator, which generates a sawtooth waveform. Here’s how to build one with the 2N6027:
Components:
PUT: onsemi 2N6027
Resistors: R1 (current limit), R2, R3 (programming resistors)
Capacitor: C1 (timing capacitor)
Load Resistor: R4
Circuit Operation:
1. Programming: The gate voltage `V_G` is set by the voltage divider formed by resistors R2 and R3 across the power supply (`V_S`). The formula is `V_G = V_S [R3 / (R2 + R3)]`. This `V_G` defines the peak trigger voltage.
2. Charging: When power is applied, capacitor C1 begins charging exponentially through resistor R1. The voltage across the capacitor (`V_CAP`) is also the anode voltage (`V_A`) of the PUT.
3. Triggering (Firing): When `V_A` (the capacitor voltage) becomes about 0.7 V greater than `V_G`, the PUT turns on.
4. Discharging: The ON PUT provides a low-resistance path, causing capacitor C1 to rapidly discharge through the PUT and the small load resistor R4. This sudden current flow creates a sharp voltage pulse across R4.
5. Reset: Once the capacitor has discharged, the anode current falls below the valley current (`I_V`) of the 2N6027. The PUT turns off, and the cycle repeats from step 2.
The output across R4 is a series of sharp pulses, while the voltage across C1 is a sawtooth wave. The oscillation frequency is primarily determined by the RC time constant of R1 and C1 and the programmed standoff ratio set by R2 and R3. The formula for the approximate period (T) is:
`T ≈ (R1 C1) ln(1 / (1 - η))`, where `η ≈ R3 / (R2 + R3)`
Key Advantages of Using a PUT like the 2N6027
Programmability: The trigger voltage is easily set with two external resistors.
High Input Impedance: The gate presents a high impedance to the programming voltage divider, making it easy to drive.
High Peak Current Pulse: Capable of discharging capacitors very quickly, producing sharp current pulses ideal for triggering other devices like SCRs and triacs.
Simple Circuit Design: Requires very few components to create a robust oscillator or trigger circuit.
The onsemi 2N6027 PUT remains a highly effective and simple solution for generating timing cycles, pulse waveforms, and triggering circuits. Its fully programmable nature through external resistors provides a level of design flexibility that its predecessor lacked. For engineers and hobbyists alike, understanding and utilizing the PUT is a valuable skill for creating simple, reliable, and efficient oscillator designs without the need for complex ICs.
Keywords:
Programmable Unijunction Transistor
Relaxation Oscillator
onsemi 2N6027
Trigger Circuit
Peak Point Voltage
